Method and apparatus for the determination of gas concentration in molten metal and metal matrix composites

ABSTRACT

The invention provides a new method for the determination of the concentration of gas dissolved in a molten metal or metal matrix composite employing a new immersion head probe in apparatus employing the method. Such determinations are needed to facilitate removal of the gas, which can cause loss of desirable properties and/or bubbles in the solidified material and subsequent processing difficulties. This determination is particularly difficult with metals containing high concentrations of particulate additions, such as metal matrix composites, since it is necessary to avoid deposition of the particulates and consequent inaccurate readings. The method employs apparatus which circulates an inert carrier gas through the probe in gas exchange contact with the molten metal to entrain dissolved gas until an equilibrium mixture is obtained; the concentration of the dissolved gas in the mixture then is representative of its concentration in the molten metal. The head consists of a monolithic or integral body of a porous gas-permeable material of sufficient porosity, pore size and permeability to permit the necessary gas diffusion in a reasonable period of time. If the test is to be carried out in a stationary body of molten metal, the probe may be vibrated or the metal may be stirred.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Ser. No. 199,673,filed May 27, 1988, now U.S. Pat. No. 4,907,440, issued Mar. 13, 1990,of Jean-Pierre Martin, Ghyslain Dube, and Don A. Doutre (three of theapplicants herein), for "Probe for Determination of Gas Concentration inMolten Metal," and assigned to the same assignee as the presentapplication.

FIELD OF THE INVENTION

The present invention relates to a method for measuring theconcentration of a gas such as hydrogen dissolved in a molten metal orin a metal matrix composite, so as to permit the total content of thegas in the metal to be determined, and to apparatus employing such amethod. More particularly, the invention is concerned with a method andapparatus for direct measurement of the content of hydrogen dissolved inliquid metal, more specifically molten aluminum and alloys thereof.

REVIEW OF THE PRIOR ART

Many metals including aluminum and its alloys when in the liquid statereact chemically quite readily with the moisture in the atmosphere toform gaseous hydrogen which, owing to its high solubility will dissolvereadily in the liquid metal. This is particularly true of aluminum andits alloys and for convenience the following discussion will makereference principally to this metal. Thus, the solubility of hydrogen inaluminum and its alloys is particularly high, about 1 mL STP/100 gramsat the melting temperature (about 700° C.), but the solubility in thesolid metal is only about one-tenth of this value, and this dissolvedhydrogen can generate serious problems during further processing of thesolid metal. For example, during solidification there is a strongtendency for the excess gas to be expelled from the metal, leading tothe formation of blow holes and gas bubbles which are trapped therein.Such bubbles lead to the formation of cracks in the cast ingots, whichcan have disastrous consequences during subsequent rolling operations,and can ruin the surface finish of thin foil products. There istherefore an increasing requirement to degas the molten metal prior tothe metal casting process. Degassing processes usually comprise theintroduction of chlorine gas and/or an inert gas such as nitrogen orargon into the molten body or stream of metal in the form of adispersion of fine bubbles. Typically dilute mixtures of chlorine inargon are used with one or more lances or rotating impellers tointroduce the degassing media into the melt. The efficient operation ofthe degassing process requires an accurate knowledge of theconcentration of the hydrogen gas in the metal, so that its totalcontent can be determined, and numerous techniques exist for suchmeasurement. Most of these techniques require the preparation of a solidsample and access to sophisticated analytical equipment suitable onlyfor use in a laboratory setting and not the relatively arduousconditions of a metal casting shop. Moreover, although these methods areprecise they are relatively slow and do not allow the necessaryinformation to be obtained "on-line" during the progress of a castingoperation.

Another group of metal products for which accurate determination of gascontent, particularly hydrogen content, is important, are metal matrixcomposites consisting of a base metal, usually aluminum, reinforced bythe incorporation therein of substantial quantities of non-metallicparticulate materials (e.g. silicon carbide, alumina, titanium diboride,etc.) uniformly distributed throughout the base metal. It is found thatthe presence of hydrogen is deleterious to the desired properties of thecomposites, and it is a standard step in the processes for theirproduction to remove the absorbed hydrogen from the molten metal. Thismay be achieved by vacuum degassing the composite melt while molten.Examples of such processes are given in U.S. Pat. No. 4,759,995, issuedJuly 26, 1988 to Dural Aluminum Composites Corporation, the disclosureof which is incorporated herein by this reference. The introduction andretention of large quantities of non-metallic particulates (up to 40% byvolume or 50% by weight) uniformly distributed in the base metal isdifficult, requiring continued agitation of the molten mixture, andrapid "on-line" determination of the gas content is therefore even moreessential.

There is at present only one method known to the applicants whichenables direct measurement within the molten metal and allows on-lineanalysis in the plant, namely the "Telegas" process, as described inU.S. Pat. No. 2,861,450 of Ransley et al. The "Telegas" apparatuscomprises a probe immersion head which is immersed in the molten metal,the head comprising an inverted collector cup or bell of heat resistantimpervious ceramic material whose mouth is closed by a ceramic filter toform a chamber within its interior. A first capillary tube extendsdownward through the head and the filter, while a second such tubeextends upward from the interior of the chamber. A fixed quantity of aninert gas, usually nitrogen, is circulated in the apparatus by feedingit down through the first tube and withdrawing it through the secondtube, so that it bubbles into the molten metal adjacent the head, thebell collecting the upwardly-moving bubbles, while the ceramic filterprevents the molten metal from entering the enclosure. The nitrogenentrains some of the hydrogen in the adjacent metal and is constantlyrecirculated for a sufficient length of time, usually about 5 to 10minutes, until the partial pressure of the hydrogen gas in thenitrogen/hydrogen mixture reaches an equilibrium value. Owing to thehigh mobility of the dissolved hydrogen in the molten metal, this willaccurately represent the hydrogen concentration throughout the body ofthe melt.

As equilibrium is approached the concentration of the hydrogen in thecarrier gas is monitored by measuring the difference in electricalresistance of two like hot-wire detecting elements disposed inrespective equal measuring cells, one of which receives thenitrogen/hydrogen mixture and the other of which has an atmosphere whosethermal conductivity is substantially equal to that of the nitrogen,usually air. The difference in resistance is measured by a bridgecircuit, the value being calibrated to correspond to the hydrogen gasconcentration value, as determined by any of the laboratory-typeanalytical apparatus mentioned above. This measured value will need tobe compensated for melt temperature, and also for the differentsolubility of hydrogen in the specific metal or alloy with which theapparatus is employed, by any of the methods well known to those skilledin this particular art.

There are several technical problems connected with this type ofimmersion head. Firstly, the probes are made of high density ceramicmaterials in order to be resistant to the molten metal and also to beimpervious to diffusion of the hydrogen therethrough, so that faultyreadings will not be obtained. Such materials have very low resistanceto thermal and mechanical shock, and any mishandling leads to damage oreven destruction. For example, it is essential in practice to preheatthe probe before immersion by positioning it close to the body of moltenmetal, and to insert it and withdraw it slowly from the metal in orderto prevent such thermal shocks. Again, such a probe theoretically shouldbe effective for 20 to 30 analyses before requiring replacement, but itis not unknown for them to become useless after only three immersions inthe melt. The usual cause of this is splashing of the liquid metalduring the part of the analysis cycle in which the gas mixture is purgedfrom the probe, this metal blocking the porous ceramic element so thatit cannot perform its function. Further, because of the design theprobes are relatively expensive to produce. Difficulties also arise inobtaining rapid and accurate analyses, owing to the particular shape ofthe probe. Thus, if the probe is not kept vertical in the molten metal,some of the carrier gas may escape from beneath the cup to the surface,leading to an erroneous reading. Moreover, the gas that bubbles from thefirst conduit ideally should disperse uniformly in the adjacent body ofmetal, but instead tends to stay close to the outside wall of theconduit, so that the recirculation time is considerably increased.

Owing to the size of the probe a minimum metal depth of 7.5 cm. (3 ins.)is required in the metal bath for effective operation. This is sometimesdifficult in the production of metal matrix composites which may involvemeasurement in the molten metal flowing in a shallow layer in a transferthrough between the furnace and the casting station. Moreover, the"Telegas" probe requires an initial period of substantial length duringwhich the carrier gas bubbles through the body of the metal adjacent theprobe; this may cause the melt to become locally inhomogeneous, at leastin the neighbourhood of the probe. The shape of the probe with itsinverted bell cavity for trapping the hydrogen makes it particularlysusceptible to the deposition of particulate material that can obstructthe gas flow and lengthen the time required for the test to be complete,and may even stop the flow completely.

Another form of immersion probe has been disclosed in a paper by R. N.Dokken and J. F. Pelton, of Union Carbide Corporation, entitled "In-LineHydrogen Analysis in Molten Aluminum" and presented in an internationalseminar on refining and alloying of liquid aluminum and ferro alloysheld in Trondheim, Norway on Aug. 26-28, 1985. The probe is also thesubject of U.S. Pat. No. 4,624,128 issued Nov. 25, 1986 to Union CarbideCorporation. This probe was intended to replace the "Telegas" probe andcorrect deficiencies perceived therein, such as the possibility that therecirculating gas forms an envelope around the tip of the probe to causea loss of carrier gas and consequent inaccuracy. This probe comprisestwo long concentric metallic tubes attached to two heavier metallictubes. The outer tubes are protected from dissolution into the aluminumby having a woven ceramic blanket covering their outer surfaces. The twoheavier tubes are the measuring head of the probe, with the spaceswithin the ceramic fiber weave providing a zone for the transfer ofhydrogen from the molten aluminum to argon carrier gas in these spaces.This carrier gas is recirculated through the two long concentric tubesup to the measuring portion of the instrument.

This probe is essentially a steel structure in which the area of thegas/aluminum exchange surface is of the same order as that of thesteel/aluminum contact surface. Hot steel at the operative temperatureis quite permeable to hydrogen and is subject to oxidation; theresulting oxidized steel can develop an exothermic reaction with themolten aluminum, and the oxide can react with the hydrogen to formwater, leading to false readings. Owing to its design, the regionsenclosed by the ceramic weave are effectively "dead" zones having littleor no direct contact with the circulating carrier gas, and there ismoreover the clear possibility of the inflowing gas "short-circuiting"directly from the inlet to the outlet, leading to longer equilibriumtimes.

DEFINITION OF THE INVENTION

It is therefore a principal object of the present invention to provide anew method for determining the concentration of gas dissolved in a bodyof molten metal, particularly to a method that provides an "on-line"direct measurement of such gas concentration, and more particularly to amethod that permits such measurement of the concentration of hydrogen inaluminum, aluminum alloys and aluminum metal matrices of compositematerials.

In accordance with the present invention there is provided a method fordetermination of the concentration of a gas dissolved in a molten metal,including the steps of:

immersing in the molten metal a probe body consisting of agas-permeable, liquid-metal-impervious material of sufficient heatresistance to withstand said immersion;

the body having a gas inlet to its interior, and a gas outlet therefrom;

the gas inlet and outlet being spaced from one another so that gaspassing from the inlet to the outlet traverses a substantial portion ofthe probe body interior for entrainment of gas diffusing to the interiorof the body from the ambient molten metal;

passing a carrier gas through the probe body interior to entrain gas tobe determined that has diffused therein from the molten metal; and

measuring the concentration in the carrier gas of the gas to bedetermined.

Also in accordance with the invention there is provided a method for thedetermination of gas concentration in a molten metal including the stepsof:

immersing in the molten metal an immersion probe comprising:

a probe body consisting of- gas-permeable, liquid-metal-imperviousmaterial of sufficient heat resistance to withstand immersion in themolten metal;

the body having a gas inlet to its interior and a gas outlet therefrom;

the gas inlet and outlet being spaced from one another so that gaspassing from the inlet to the outlet traverses a substantial portion ofthe probe body interior for entrainment of gas diffusing to the interiorof the body from the ambient molten metal;

and recirculating a carrier gas in a closed circuit between the probeand a gas concentration determining means adapted to determine theproportion of the gas present in a mixture thereof with the carrier gas.

Preferably, the gas to be detected is hydrogen and the metal is selectedfrom aluminum, aluminum alloys and aluminum matrix composites.

DESCRIPTION OF THE DRAWINGS

Particular preferred embodiments of the invention will now be described,by way of example, with reference to the accompanying diagrammaticdrawings, wherein:

FIG. 1 is a schematic diagram of an apparatus including a probe devicefor measuring the gas content of a molten metal;

FIG. 2 is a cross-section to a larger scale of the body of the probedevice of FIG. 1, taken on the line 2--2 of FIG. 3;

FIG. 3 is another cross-section view of the probe device body taken onthe line 3--3 of FIG. 2;

FIG. 4 is a cross-section similar to FIG. 2 of another form of immersionprobe body of the invention;

FIGS. 5 through 12 are similar elevational views of differentconfigurations of probe bodies of the invention;

FIGS. 13 through 15 illustrate different arrangements of the probedevice to increase contact between the probe body surface and the liquidmetal;

FIGS. 16 through 18 are graphs of test results for different alloysemploying the probe of the invention; and

FIGS. 19 and 20 are graphs of test results employing the method andprobe of the invention with various aluminum matrix composites.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 there is shown therein a probe element 10 of theinvention, consisting of a monolithic body 12 of gas-permeable,liquid-metal-impervious material, immersed in a body 14 of molten metal,specificallY of molten aluminum or an alloy thereof, or a compositethereof. The body 14 may be stationary, as would be obtained in a ladleor a laboratory sample, or it may be a stream of metal, as would beobtained in a transfer trough leading from a casting furnace. Thespecific structure of the probe element will be described in detailbelow. A fine bore tube 16 extends from a gas inlet 18 in the body ofthe probe element to a recirculation pump 20 via a non-return valve 22,and thence via another non-return valve 24 to the gas outlet of thesensing cell 26 of a katharometer 28. Another fine bore tube 30 extendsfrom a gas outlet 32 from the body 12 to the gas inlet to thekatharometer sensing cell 26, so as to complete a closed circuitincluding the probe element, the pump and the cell. The tube 30 includesa T-junction by which the gas circuit is connected to a controllableflushing valve 34 which when opened admits a flushing gas, usuallynitrogen, into the circuit from a suitable source, usually a cylinder ofthe compressed gas (not shown).

In the embodiment of FIG. 1, the comparison cell 36 of the katharometeris open to atmosphere, since ambient air is a suitable comparison mediumwhen the carrier gas is nitrogen. However, if some other carrier gas isused, such as argon, it would then be necessary either to seal thecomparison cell containing said gas, or to flow the gas continuouslythrough the cell. Each cell contains a respective fine resistance wire38 and 40 connected as the respective adjacent arms of a bridge circuit42. The other bridge arms are constituted in well known manner byresistors 44 and 46, the bridge is supplied with operating current frombattery 48 via adjusting resistor 50, and a bridge meter 52 or othermeasuring device being connected in known manner between the twoopposite junctions. A thermocouple 54 is mechanically connected to theprobe element 10 so that it is immersed therewith into the molten metal14 and provides the necessary measurement of the metal temperature.

The thermocouple 54, the pump 20, the flushing valve 34, and the bridgemeasuring device 52 are all connected to a computer controller 56 whichis arranged to automatically control the apparatus through eachconcentration determining cycle of operations, and to feed the resultsof the cycle to one or more display and/or recording devices which willbe apparent to those skilled in the art.

A typical measurement cycle will begin with the flushing valve 34 beingopened by the controller 56, so that dry nitrogen under pressurecirculates through the entire circuit, entering at both the probe gasinlet 18 and the outlet 32 and exiting through the porous body of theprobe element; this circulation is maintained long enough to ensure thatonly nitrogen remains in the circuit. On start-up it is also desirableto maintain the flushing for a sufficiently long period to ensure thatall moisture has been eliminated. The flushing operation is maintaineduntil the probe has been lowered into the melt when the valve 34 isclosed and the pressure of the nitrogen in the circuit will quicklyreach a steady value. In practice the flushing is carried out at a gaspressure of about 20 to 50 KPa (3 to 7 p.s.i.), which reduces to a rangeof about 2 to 8 KPa (0.25 to 1 p.s.i.) during the test procedure. Theoperation of the pump motor causes the volume of carrier gas in thecircuit to be constantly recirculated therein, passing in the body 12from the inlet 18 to the outlet 32.

Owing to the very high mobility of hydrogen in liquid aluminum at theusual temperatures involved (about 700° C.), it will rapidly and easilyenter the porous probe body in attempting to establish concentrationequilibrium and become entrained in the carrier gas, the circulation ofthis gas being maintained for a period of time known to be sufficient toestablish equilibrium, usually of the order of 1 to 10 minutes. At theend of this period the controller is operative to take a measurement ofthe difference in resistivity of the resistance wires 38 and 40 in thekatharometer. The nitrogen/hydrogen mixture causes increased cooling ofthe wire 40 because of the presence of the hydrogen, this increase beinga measure of the partial pressure or concentration of the hydrogen inthe nitrogen/hydrogen mixture, and thus of the concentration of thedissolved hydrogen in the metal body. The controller will usually bearranged to compute the concentration value directly, as will beapparent to those skilled in the art, including the application of acorrection factor from an operator-adjusted circuit 58 to account forthe different solubility of hydrogen in different metals and alloys.Upon conclusion of the measurement portion of the cycle the circuit isflushed by a short burst of argon gas to purge hydrogen from the system,so that it is ready for a new cycle. The probe may be removed from themetal or left in place at the choice of the operator.

The improved operation of the probes of the present invention is bestdescribed by comparison with the "Telegas" probe which consists of adense gas-impervious ceramic body from which the nitrogen carrier gas isbubbled into the metal body in direct contact with the metal and thehydrogen dissolved therein. It has been considered necessary for suchdirect contact to take place to obtain effective entrainment of thehydrogen in the carrier gas. Some of the difficulties obtained inpractice with this apparatus have been described above, and itsapplication to the measurement of hydrogen content in metal matrices ofcomposite materials is even more difficult. The large quantities ofparticulates in such composites result in physical properties andbehaviour entirely different from that of the metal and its usualalloys,.primarily differences in viscosity and density, with the resultthat meaningful and reproducible readings become more difficult toobtain. In addition to the deposition and depth problems discussedabove, a preheating period of at least 10 minutes is recommended for the"Telegas" probe before it is inserted into the molten metal, to avoidexcessive thermal shock to the high density ceramic from which it ismainly fabricated. Owing to the need to keep the particulate componentin suspension, and the typically small size of production batchescurrently in use, the casting time for composites is relatively shortand this relatively long preheat period makes it unusable for thepurpose.

By contrast a probe element 10 of the invention, by elimination of thisbubbling and its replacement with direct diffusion and mixing of thegases within the interstices of the probe body, can consist of a singlemonolithic or unitary block of material of suitably chosen porosity,pore size and permeability provided with a gas inlet and a gas outletspaced sufficiently apart that the circulating carrier gas must traversea substantial portion of the interior of the probe body. The small probebody almost immediately reaches the temperature of the ambient metal,and the hydrogen therefore readily diffuses in the pores of the block,so that it will quickly mix with the carrier gas and attain thenecessary equilibrium of concentration.

The elimination of the bubbling period with the method and probe of theinvention means that when used with metal matrix composites theparticulates in the metal matrix are not disturbed and do nor separateand rise to the surface while the hydrogen is being measured. Many ofthese particulates have higher densities than liquid aluminum andprevention of such separation is one of the principal difficult problemsin this technology. There is therefore no change in the mixture, whichremains homogeneous and the quality of the product is preserved. Sincethe porous, cavity-free probe is not wetted by the liquid metal theproblems of agglomeration are prevented, and obstruction by theparticles is unlikely to arise. Because of its small size the probe canoperate successfully in very shallow vessels or transfer troughs, with adepth as little as 2.5 c.m. (1 in.).

The porosity of a body is usually expressed as a percentage and issimply the proportion of the total volume of the body that is occupiedby the voids within the body, a highly porous body having a highpercentage of voids. A high porosity has the advantage that the materialis usually more resistant to thermal shock, so that the probe can beplunged directly into the metal without preheating, and removed withouthaving to cool it slowly, making it particularly suitable for metalmatrix composite production processes which typically employ very shortcasting times of about 10 to 20 minutes. There is moreover greateropportunity for diffusion of the hydrogen into the body, circulation ofthe nitrogen in the body, and mixing of the two gases together. However,a high porosity body inevitably has many large pores and is usuallystructurally weaker, to the extent that it may be difficult to anchorthe tubes 16 and 30 in the body, and the probe may become too fragilefor satisfactory handling under industrial testing conditions. Again,because of the large pores of a highly porous body difficulty may beencountered in the liquid metal seeping into the body. The range ofporosity for the probe bodies of the invention is from a minimum ofabout 5% to a maximum of about 80%, but preferably is in the range ofabout 20% to about 60%, and more preferably is in the range from about35% to about 40%.

A second important consideration in the choice of suitable materials forthe probe body is the pore size, and this can vary over a wide range,namely from about 0.5 micrometers to 2,000 micrometers, since the sizeof the hydrogen molecules in the metal is of the order of 2×10⁻⁴micrometers (2 Angstroms), and both gases can diffuse easily even in thesmallest size pores. The lower limit is determined more by the impairedresistance of fine-pored materials to thermal shock, while the upperlimit is dictated by mechanical assembly problems, as described aboveand the increased possibility of the molten metal entering the largerpores. For example, with aluminum under normal operating conditionspenetration of the metal into the pores will start to become excessiveabove 1,000 micrometers. The preferred pore size is therefore in therange 10 micrometers to 1,000 micrometers, and more preferably is in therange 100 micrometers to 250 micrometers.

The third important consideration in the material choice is itspermeability. A body of porosity and pore size within the preferredranges may still be unsatisfactory if the cells or voids are completely"closed" off from one another, or are so poorly interconnected that thegases cannot diffuse and mix together within a reasonable period oftime.

As previously described, the porosity of the probe body must be duepredominantly to interconnected pores or voids so that it issufficiently permeable to the gases. Permeability may be generallydefined as the rate at which a gas or liquid will pass through amaterial under a specified difference of pressure. Permeability of anygiven material can be measured by determining the quantity of a fluid(in this case air) that will flow through a thin piece of the materialof specified dimensions under a specified low pressure differential.

For flows occurring under low pressure differentials, D'Arcy's Lawstates: ##EQU1## where Q=Air flow (m³ /s)

P_(e) =Specific permeability (m²)

L=Sample thickness (m)

A=Sample cross-sectional area (m²)

u=Air viscosity at the temperature of measurement (1.84×10⁻⁵ Kg/m-s at20° C.)

P=Pressure (Pa)

The permeability is usually expressed in Darcy units, where:

    1 Darcy=1×10.sup.-12 m.sup.2

Therefore equation (1) can be written: ##EQU2## where P_(D) is thespecific permeability expressed in Darcies.

For air at 20° C. and using a pressure differential of 2 in. H₂ O (500Pa): ##EQU3##

With the probes of the invention it is preferred that the permeabilitybe in the range about 2 to about 2,000 Darcies, more specifically in therange about 10 to about 100 Darcies.

The pore size of the material must be such that both of the carrier gasand the hydrogen will diffuse readily therethrough and become mixed withone another, while it must be impossible for the metal to enter morethan the surface layer of the probe body. Thus, it is acceptable to findafter the conclusion of a measurement cycle that a thin skin ofsolidified metal has mechanically adhered to the exterior surface of theprobe, since this can readily be stripped away before the next cyclewithout damage to the probe. Theoretically, it would seem to beadvantageous for the exterior surface of the probe body to bemetal-wettable, so as to obtain a high-diffusion interface between themetal and the probe, but in practice it is found that reproducibleresults can be obtained with a monolithic body of non-wettable material,particularly if the probe and/or the metal are stirred as describedbelow. The presence of the above-described thin skin of aluminum on theprobe surface indicates that the surface has become wetted and once thishas taken place the surface will remain wetted. Wetting can befacilitated by precoating the body with a thin layer of a suitable metalsuch as aluminum, silver, nickel or platinum, as indicateddiagrammatically in FIGS. 2 and 3 at 59 by the broken outline. The metallayer can be applied by any of the well-known processes for suchdeposition such as dipping, spraying, electrolylic, electroless, etc.,the layer being preferably of about 10 micrometers (0.0004 in) to 1000micrometers (0.04 in) in thickness.

It is found particularly advantageous to employ for the coating 59 amaterial that has a catalytic action toward the hydrogen, promotingassociation from its monatomic state in the molten aluminum to themolecular diatomic state in the probe body for its entrainment in thecarrier gas. A particularly suitable metal for this purpose is platinum,which can readily be deposited in the desired very thin layers fromcommercially available electroless platinising solutions. Because of itsmetallic nature platinum will in addition facilitate wetting asdescribed above. As an example of a suitable process the body 12 isimmersed in the platinising solution for a brief period which may befrom about 5 seconds to about 5 minutes (the specific time dependingupon the solution concentration and the coating thickness desired), thesolution normally consisting of about 3% concentration of platinumchloride (PtCl₄) or hydroplatinochloride (H₂ PtCl₄) in hydrochloricacid, optionally including lead acerate as a buffer. The body is thenbaked at a temperature above 500° C., usually about 800° C., to ensurethat no residual hydrochloric acid remains. The coating obtained isestimated to be of thickness of about 1 micrometer (0.00004 in.) to 100micrometers (0.004 in.) and thicknesses of about 0.1 micrometer(0.000004 in.) to 1000 micrometers (0.04 in.) are considered to besuitable. It is found that in use the catalytic coating does eventuallydissolve away and if the probe body still has sufficient useful life itcan easily be re-coated. Other materials that will function in thismanner are, for example, palladium, rhodium and nickel.

The shape of the probe is not at all critical, but it is advantageousthat in at least one dimension it be as small as is practical, so as toprovide a corresponding minimum path length for the hydrogen to diffuseinto the block interior. Preference is also given to shapes thatmaximize the active metal/probe surface area for a given probe volume.These considerations give preference to the shape of a thin wafer, asillustrated by FIGS. 2 and 3, that is rectangular in all elevations. Itwill be noted that wherever possible edges of the body are rounded so asto avoid as much as possible sharp corners that are particularlysusceptible to mechanical shock. The thickness of the probe to providethe desired minimum path length should be between about 0.5 cm and 1.5cm, the minimum value being determined also by the mechanical strengthof the material and thus of the resultant wafer. Advantageously thevolume of the probe is between 1 cc and 10 cc, preferably from 2 cc toabout 5 cc.

Referring again to FIGS. 1 to 3, it will be seen that in this particularembodiment the probe body 12 is provided with two parallel bores 60 and62 which respectively receive the ends of the two tubes 16 and 30; thebores extend into a groove 64 in which the tubes are bent to lie andinto which they are fastened by a layer of a suitable heat resistantcement 66 (FIG. 1). This structure brings the two tubes closer together,as seen in FIG. 1, to facilitate their enclosure in a sheath 68 of aheat resistant material, such as a material woven from an alumina fibre,and at the same time provides added resistance to torques that areapplied to the body during its handling and its immersion, etc. in thebody of liquid metal.

In constructing an apparatus of this type it is desirable to keep thevolume of carrier gas that is required as small as possible, so as todecrease the time required for equilibrium to be reached, and thisconsideration dictates the use of narrow bore tubes 16 and 30, aminiature recirculating pump 20 and a probe 10 of small volume. It willbe understood that the volume of gas to fill the probe will be at mostthe volume of the voids therein. A practical volume for a completesystem is between 1 cc and 5 cc, while a practical gas flow rate toobtain a reasonably short response time is from about 50 cc to about 200cc per minute. However, as the volume of the probe is reduced there is acorrespondingly reduced access of the metal and the hydrogen in the meltto the carrier gas and a compromise is therefore necessary. A verysuccessful probe of the invention consists of a porous circular-segmentalumina disc as shown in FIG. 4 of porosity about 35% to 40%, averagepore size about 120 micrometers and permeability about 25 Darcies. Thebody has a thickness 0.64 cm (0.25 in.) and diameter 2.5 cm (1.00 in.)to have a volume of about 3 cc (0.3 inch cubed).

It will be seen that a simple monolithic block of such shape is easy tomanufacture by well known procedures. Because of its compactconfiguration, such a body inherently has high resistance to mechanicalshock. Moreover, since it is operative totally immersed in the liquidmetal with the exchange of hydrogen between probe and metal taking placethrough the probe body surface, and the hydrogen entrainment into thecarrier gas taking place entirely within the interior of the probe body,then its atitude and positioning in the metal body is completelynon-critical avoiding this possibility of error. It will also be notedthat because of this internalization of the mixing or entrainmentmechanism the probe is able to operate successfully in a fast-movingstream of metal, such as in a transfer trough, which is not the casewith a probe relying on external bubbling for entrainment, when thebubbles may be swept away before they can return into the probe. Thematerial must be refractory in nature, namely able to withstand thetemperature of immersion without softening to an unacceptable degree,and as non-reactive as possible with the metal, since such reactivitywill eventually require the probe body to be replaced. A verysatisfactory probe material for use in aluminum is fused granularalumina, the grains being held together by a porcelanic bond; suchmaterials of a wide range of porosities are commercially available.

It will be seen that the probes of the invention can easily be madeentirely of non-metals, avoiding problems of corrosion and diffusion ofthe hydrogen, which at the temperatures involved will diffuse throughmost commercially useful metals. By suitable choice of the porousmaterial used for the body it is possible to obtain a large gas exchangesurface in a compact monolithic or unitary integral body, with a maximumof the body volume occupied by the pores and minimum of "dead volume"occupied by the solid material.

The probes of the invention can take a number of different forms, andsome examples are shown in FIGS. 4 through 12. As previously described,the embodiment of FIG. 4 is formed as the major segment of a flatcircular disc, while that of FIG. 5 is a complete circular disc, thetubes 16 and 30 extending different distances into the body 12 toincrease the length of the flow path between the inlet 18 and outlet 32.FIG. 6 shows a rectangular body that is somewhat longer than it is wide,with the tubes 16 and 30 extending different distances into the body, aswith the structure of FIG. 5, while FIG. 7 shows a probe with acylindrical body, the tubes 16 and 30 entering at opposite ends. FIG. 8illustrates a triangular-shaped probe body and FIG. 9 anelliptical-shaped body, while FIG. 10 shows that a quiteirregular-shaped body of a suitable material can be provided with a gasinlet and outlet and function successfully. FIG. 11 illustrates the factthat the body is not necessarily monolithic, i.e. formed from a singleblock of material, but instead can be an integral body that is assembledfrom more than one piece joined together by a suitable cement (notshown), care being taken to ensure that the cement layer does notconstitute a barrier to free diffusion of the gases through the bodyfrom the inlet to the outlet. The bores 60 and 62 are in this embodimentconstituted by mating semi-circular cross-section grooves. FIG. 12illustrates another integral structure containing a large open void 70into which the tubes 16 and 30 discharge, the hydrogen diffusing intothis volume through the wall of the probe body; such a structure doespermit a somewhat less porous material to be used for the body, sincehydrogen diffuses more easily than nitrogen and only the hydrogen needsto diffuse through the body. The size of the void 70 should not be suchthat it increases substantially the response time of the probe.

The probes of the invention have been described in connection with thedetermination of hydrogen concentration in aluminum and its alloys, butcan of course be used for the determination of this and other gases inother metals, such as magnesium, copper, zinc, steel and their alloys.

There is a wide range of manufactured and naturally-occurring materialsthat can be used to form an immersion probe of the invention, providedof course that upon test they are able to meet the requirement of thecombination of mechanical strength, porosity, pore size andpermeability. Examples of synthetic materials are:

(a) Porous ceramics that are sufficiently refractory in nature to beused with the metal under test, including the carbides, nitrides andoxides of aluminum, magnesium, silicon, zirconium, tungsten andtitanium;

(b) Ceramic foams and fibres;

(c) Grinding materials and synthetic minerals, particularly thesilicates and spinels;

(d) Composites of fibres in metal matrices;

sintered metal powders of sufficiently high melting point, e.g. steel,titanium and tungsten; since such materials are metal-wettable theyshould be provided with a gas-permeable coating of a metal non-wettablematerial;

(e) Porous graphite and other carbon based materials, including fibresof such materials in mat form or embedded in a suitable matrix; and

(f) Filtered porous glasses of sufficiently high melting point, such aspyrex and aluminosilicates; porcelains.

Examples of naturally-occurring materials are mullites, sandstones, andpumices. The materials can be prepared to have the necessary propertiesand shape by any of the well known techniques, such as sintering,pressing, binding, gas forming, moulding, drilling, grinding, etc.

When use of the probes of the invention involves their immersion in amoving stream of metal, the movement of the metal past the probe(typically of the order of 5 cm/sec) ensures adequate contact betweenthe probe surface and the metal to obtain a reasonably short responsetime to nitrogen/hydrogen equilibrium. However, as with any probe thisperiod is increased if the bath is static. Owing to the inherentstructure of the probes it is possible to shorten the test time in astatic bath by creating an artificial relative movement between theprobe and the metal. This is not possible with prior art probes usingexternal bubbling because of the danger of loss of the circulatingcarrier gas if it does not remain sufficiently close to the probe to berecaptured thereby. Thus, it is found that the response time with theprobes of the invention can be reduced to values of about 2 to 5 minutesby use of the embodiments illustrated by FIGS. 13 to 15.

With the apparatus of FIG. 13 the probe element 10 is mounted on avibrator 72, the movements of the probe produced by the vibrator 72facilitating the diffusion of the hydrogen across the probe/metalinterface. The vibrator can be of mechanical or magnetostrictive typeand vibrates the probe in any mode that it produces.

With the apparatus of FIG. 14 the probe is mounted to rock about a pivot74 under the action of a motor-driven eccentric 76 connected to theprobe support by a shaft 78. With both systems the range of movement ofthe probe is preferably in the range 0.5 to 5 Hertz, more preferably inthe range 1 to 2 Hertz, and with a mechanical excursion in the range 10to 100 mm.

With the apparatus of FIG. 15 the probe is stationary during the test,and instead the molten metal is circulated around the probe by means ofa small impeller 80 driven by a motor 82, this circulation againfacilitating diffusion at the probe/metal interface. An impeller ofabout 8 cm diameter rotating at speeds in the range of 100 to 400 r.p.m.is found to be completely effective.

To determine the effectiveness of the probes of the invention 28different probes were employed in comparison tests that were confirmedusing existing laboratory instruments. Each probe was tested understatic conditions for three repeat measurements being taken out of themetal bath, comprising a small laboratory furnace at temperatures from700° C. to 750° C., between each test. The values obtained ranged from0.05 to 0.45 ml/100 g, with most values in the range 0.15 to 0.25 ml/100g for four different alloy types, namely:

(a) commercially pure aluminum (99.5%);

(b) aluminum/magnesium alloys-including up to 5% by weight Mg;

(c) aluminum/zinc/magnesium alloys including up to 5% by weight Zn andup to 2% Mg

(d) aluminum/lithium alloys including up to 3% by weight Li

The overall probe to probe reproducibility (84 values) was 0.017 ml/100g, while the average repeatability of the same probe was 0.012 ml/100 g.The usual response time under these static conditions was 8 to 10minutes. The precision of these values may be compared with thereproducibility values of 0.03 to 0.05 ml/100 g obtained with a nitrogencarrier fusion laboratory-type analyser.

FIGS. 16 through 18 are test results obtained with the following metals:

FIG. 16: Unalloyed aluminum at 705° C.

FIG. 17: Al/Zn/MG alloy with 5% Zn and 2% Mg at 709° C.

FIG. 18: Al/Li alloy with 2.5% Li at 720° C.

The reproducibility of all of the results will be noted. Adequateequilibrium for testing was reached with the unalloyed aluminum in 5minutes with an acceptable value at 4 minutes. The results obtained withthe Al/Zn/Mg alloy were even faster with acceptable equilibrium at alittle over 2 minutes and complete equilibrium at 3 minutes. Completeequilibrium was reached with the Al/Li alloy in 2 minutes, with thereproducibility differing the most, namely over the range 0.26 to 0.29ml per 100 g. Lithium alloys are difficult to test with conventionallaboratory methods. In most laboratory test procedures as a solid sampleof the alloy is heated to a temperature sufficient to release thehydrogen the lithium also is released and good reproducibility iscorrespondingly difficult to obtain. Its alloys therefore requirespecial handling.

A series of experimental tests was carried out using the method andprobe of the invention for the determination of hydrogen concentrationin aluminum composites, consisting of the aluminum alloys A356, 1100,2014 and 6061 having incorporated therein from about 5% to 40% by volumeof two different reinforcing particulates, namely silicon carbide (SiC)and aluminum oxide (Al₂ O₃). The capacity of the production unit was 100kg. (220 lbs.) and the casting time employed was about 15 minutes withno preheating of the probe above the metal, the depth of metal in thecasting box being 5 cms. (2 ins.). The "Telegas" probe could not be usedbecause of the shallow casting box, and because the short casting timedid not permit the necessary preheating of the probe.

Other tests were carried out on metal composite materials in a 50 kg.(110 lbs.) laboratory unit crucible, employing the two alloys A356 and6061, and again incorporating the silicon carbide and aluminumparticulates, but in the smaller amounts of about 10% to 15% by volume.FIG. 19 shows by a solid line the results of hydrogen measurements takenwith alloys 1100 and 6061, these results being reproducible. Duringthese measurements Ransley samples were taken and the absolute hydrogencontent of the solidified metal measured by subfusion as described by C.E. Ransley and D. E. J. Talbot in J. Inst. Met. No. 84, 1955-56, pages445-452. The results are shown by the broken line in FIG. 19 andcorrelate well with an average error of 0.02 mL./100 g., using thesubfusion method as the reference value by convention.

Some of the results obtained using the method of the invention are shownin FIG. 20 by a solid line, with a comparison being shown by a brokenline of measurements carried out by nitrogen carrier fusion, whichalthough a satisfactory test method is not considered to be as accurateas subfusion with Ransley samples. A description of the nitrogen carrierfusion method is given in an article by F. Degreve in J. Inst. Met , No.47, March 1975, pages 21-26. The average difference between these twomeasurements is 0.03 mL/100 g.

We claim:
 1. A method for the determination of gas concentration in amolten metal including the steps of:immersing in the molten metal animmersion probe comprising: a probe consisting of a gas-permeable,liquid-metal-impervious material of pore size of from 0.5 micrometers to2,000 micrometers and of sufficient heat resistance to withstandimmersion in the molten metal; the body having a gas inlet to itsinterior and a gas outlet therefrom; the gas inlet and outlet beingspaced from one another so that gas passing from the inlet to the outlettraverses a substantial portion of the probe body interior forentrainment of gas diffusing to the interior of the body from theambient molten metal; recirculating a carrier gas in a closed circuitbetween the probe body interior and a gas concentration determiningmeans adapted to determine the proportion of the gas present in amixture thereof with the carrier gas so as to entrain gas to bedetermined that has diffused into the probe body interior from themolten metal and continuing the recirculation for a minimum period oftime sufficient to establish equilibrium between the concentration ofgas to be determined in the metal and in the carrier gas; and after thesaid minimum period measuring with the gas concentrating determiningmeans the concentration in the carrier gas of the gas to be determined.2. A method as claimed in claim 1, wherein the gas to be determined ishydrogen and the metal is selected from aluminum, aluminum alloys andaluminum matrix composites.
 3. A method as claimed in claim 1, whereinthe probe body is a monolithic block of material.
 4. A method as claimedin claim 1, wherein the probe body is an integral block of material. 5.A method as claimed in claim 1, and including the step of introducing acarrier gas into the closed circuit and flushing the closed circuit withthe carrier gas to remove the said gas mixture therefrom.
 6. A method asclaimed in claim 1, wherein the probe body has a porosity of from about5% to about 80%.
 7. A method as claimed in claim 6, wherein the probebody has a porosity of from about 20% to about 60%.
 8. A method asclaimed in claim 7, wherein the probe body has a porosity of from 35% to40%.
 9. A method as claimed in claim 1, wherein the probe body has apermeability of from about 2 to about 2,000 Darcies.
 10. A method asclaimed in claim 9, wherein the probe body has a permeability of fromabout 10 to about 100 Darcies.
 11. A method as claimed in claim 1,wherein the probe body has a pore size of from 10 micrometers to 1,000micrometers.
 12. A method as claimed in claim 11, wherein the probe bodyhas a pore size of from 100 micrometers to 250 micrometers.
 13. A methodas claimed in claim 1, wherein the probe body has a volume of from about1 cc to about 10 cc.
 14. A method as claimed in claim 1, wherein theprobe body is smaller in one dimension than in the other two dimensionsto provide a correspondingly shorter gas diffusion path.
 15. A method asclaimed in claim 14, wherein the thickness of the probe body in the saidsmaller dimension is from about 0.5 cm to about 1.5 cm.
 16. A method asclaimed in claim 1, and including the step of moving the probe body andthe metal relative to one another.
 17. A method as claimed in claim 16,wherein the probe body and the metal are moved relative to one anotherby vibrating or rocking the probe in the metal.
 18. A method as claimedin claim 1, and including the step of stirring the molten metal adjacentthe probe body exterior to facilitate diffusion of gas from the metalinto the probe.
 19. A method as claimed in claim 1, wherein the exteriorsurface of the probe body is coated with a thin coating of a metal tofacilitate its wetting by the molten metal.
 20. A method as claimed inclaim 19, wherein the metal is of thickness from about 10 micrometers(0.0004 in.) to 1000 micrometers (0.04 in.).
 21. A method as claimed inclaim 1, wherein the exterior surface of the probe body is coated with athin coating of a material catalysing the conversion of monatomichydrogen in the molten metal to diatomic hydrogen in the probe interior.22. A method as claimed in claim 21, wherein the coating is of thicknessfrom about 0.1 micrometer (0.000004 in.) to 1000 micrometers (0.04 in.).23. A method as claimed in claim 22, wherein the material is a metalselected from platinum, palladium, rhodium and nickel.